This invention relates to a disk refiner for ligno-cellulosic materials, and generally to disk refiners used for producing fiberboard and mechanical pulps for medium density fiberboard (MDF), thermomechanical pulps (TMP) and a variety of chemi-thermomechanical pulps (CTMP), which are collectively referred to as mechanical pulps and mechanical pulping process. In particular, this invention relates to steam flow through disk refiners in mechanical pulping processes.
A disk refiner may be used in a thermo-mechanical pulping (TMP) refiner in which the pulp material, such as wood chips, is ground in an environment of steam between a rotating grinding disk (rotor) and a stationary disk (stator) (or a pair of rotating disk rotors) each with radial grooves that provide the grinding surfaces. The rotor may operate at rotational speeds of 1000 to 2300 revolutions per minute (RPM).
Wood chips are fed to the center of the opposing disks of a disk refiner. The chips are broken down between the disks as centrifugal force pushes the chips towards the disk outer circumference. The refiner plates generally include a pattern of bars and grooves which provide repeated compression actions on the chips. The compression action results in the separation of lingo-cellulosic fibers out of the raw chips. The fiber separation transforms the raw chip material into fiber pulp suitable for a final product, such as fiberboards.
While the chips are retained between the disks, energy is transferred to the chips via the refiner plates attached to the disks. The energy is in the form of high centrifugal and compression forces applied to break-down the wood chips. The refining process also generates high frictional forces that causes water in the chip feed material to convert to high pressure steam.
In most disk refiners, the steam from the disk refiner flows in the same direction, e.g., radially outward from between the disks, as the fiber material exiting the refining disks. By way of example, typically between 60% and 100% of the steam produced between the disks in a refiner flows in a forward direction, which is the same direction as the fiber material moving between the refining disks. These percentages for forward flowing steam vary depending on refiner plate patterns and process conditions. After exiting the outer periphery of the fiber disks, the forward flowing steam carries fiber pulp through blow lines downstream of the disk refiner. The pressure of the forward flowing steam is released as the refined fiber pulp material exits the blow lines and enters bins and other relatively low pressure vessels. In MDF, the forward flowing steam typically adds little value to the pulping process and the pressure energy in the forward flowing steam is generally not used. In mechanical pulping, some systems allow for the recovery of heat energy in the forward flowing steam from a discharge cyclone, and other systems vent the forward flowing steam to atmosphere. When recovered such as via a heat exchanger, the heat from forward flowing steam from the mechanical refining processes is typically used for paper machine dryers and on pulp drying equipment
High pressure steam is needed in the feeding side of the refiner in MDF and other mechanical pulping systems. Steam is used to soften the wood to improve the performance of the refiner and produce fiber. High pressure steam for refining is usually provided a combination of back-flowing steam from the refiner and fresh steam, usually generated by a boiler. Fresh steam is expensive to produce in terms of energy consumption. There is a long felt need for sources of high pressure steam for pulping processes.
A source of high pressure steam is the steam generated during mechanical refining. High pressure steam is generated between refining disks in a disk refiner. In a traditional refiner, up to 40% of the high pressure steam generated between does not flow in a forward direction with the chip feed material. To the extent that the high pressure steam between the disks can be extracted without loss of pressure, the high pressure steam may be directed to a steaming vessel in a chip feed system of a mechanical refining plant.
A known technique to capture high pressure steam from the disks is to allow the steam to back flow against the movement of chip material between the refining disks and through the feeding system to the chip pre-steaming vessel. High pressure back flow steam has been used in the pre-steaming vessels. Separate piping has been added to refiners to allow back flow steam to bypass the conveyors and feeding devices from the feeding system, and allow the back flow steam to move with little resistance from the refiner inlet to the pre-steaming vessels.
The amount of back flow steam is generally reduced by the use of directional (low energy) refiner plates. Low energy plates typically reduce steam generation by 10 to 50% in a refiner and reduce the amount of back flow steam by 20 to 70%, as compared to conventional higher energy refiner plates. While directional MDF refiner plates are advantageous in reducing the energy required to drive a disk refiner, the reduction in the available back flow steam increases the amount of high pressure steam needed for a mechanical refining plant.
There is a long felt need for techniques to reduce the amount of high pressure steam needed to be produced at high energy costs for a mechanical refining plant. In particular, there is a long felt need to capture a greater amount of high pressure steam from the refining process than is presently captured using directional (low-energy) refiner plates in mechanical refining plants.